Parse {{define}} blocks during template parsing rather than separately as a set-specific thing.
This cleans up set parse significantly, and enables the next step, if we want, to unify the
API for templates and sets.
Other than an argument change to parse.Parse, which is in effect an internal function and
unused by client code, there is no API change and no spec change yet.

R=golang-dev, rsc, r
CC=golang-dev
https://golang.org/cl/5393049